http://code.google.com/p/dart/source/list
http://code.google.com/p/closure-compiler/source/list
http://code.google.com/p/closure-library/source/list
Their development is very much insular. That's not to say it's a poor idea to depend on these products, but they don't compare to projects like Python with formal processes for revisions (PEP) or at least some discussion.
It would be nice if Google maintained mirrors on Github and accepted pull requests and such, but I don't see that ever happening.